From what I understand, the semi-major axis of a circular orbit should be equal to its radius.
However, checking Wikipedia's info on Hubble, which is in a nearly circular orbit, I find :
sMA $= 6,919$ km
apogee $\approx$ perigee $\approx 540$ km
I'm having trouble understanding that difference.
Assuming I approximate an orbit as being circular, how can I get its radius from the semi-major axis?
Screenshot from https://en.wikipedia.org/wiki/Hubble_Space_Telescope